There are a number of organic compounds which are known to be useful as photoconductors for electrophotographic photosensitive compositions. Some of these compounds are known to have considerably high light sensitivity. However, these organic photoconductors are not practically used for electrophotographic materials.
Organic photoconductors have many excellent properties as compared to inorganic photoconductors and are widely applicable to techniques in the technical field of electrophotography. For example, the production of transparent electrophotographic photosensitive films, flexible electrophotographic photosensitive films, or light and easily handling electrophotographic photosensitive films has been made possible by using organic photoconductors. Organic photoconductors also have such properties as a film-forming property with respect to the preparation of electrophotographic photosensitive materials. In addition, they are useful in obtaining surface smoothness, and selectivity of charging polarity when applied to an electro photographic process. These properties and effects can not be obtained using inorganic photoconductors.
In spite of these excellent properties, organic photoconductors have not yet been widely used in the technical field of electrophotography mainly because of their low light sensitivity and the brittleness of films formed by such organic photoconductors.
The first study of organic photoconductors was made on compounds such as heterocyclic compounds having a low molecular weight, nitrogen-containing aromatic compounds, and various high molecular aromatic compounds. Thus, some compounds having considerably high sensitivity have been found. Recently, a sensitization method has been investigated in order to obtain higher sensitivity. Such studies have been made because even the organic photosemiconductor compounds having the highest sensitivity do not have enough sensitivity to make them practically useful without need of a sensitization treatment. Therefore, when using organic photoconductors, it is necessary to select and apply a most effective sensitizing method. Furthermore, the industrial value of organic photoconductors clearly depends upon the extent of the sensitivity of the electrophotographic photosensitive material finally obtained by the employed sensitizing method.
The most generally known sensitizing methods are the addition of sensitizing dye and the addition of Lewis acid. These methods can be applied to almost all organic photoconductors. In the former method, high sensitivity is obtained by imparting the spectral absorbing character of a dye to an organic photoconductor and in the latter method, high sensitivity is obtained by the appearance of new spectral sensitivity by the formation of a complex of a donor and an acceptor in an organic photoconductor.